The invention relates to a method for determination of the risk for disturbance-free operation of a frequency converter which is connected to a feeding mains system.
In the case of frequency converters, in particular in the case of those in which diode rectifiers are used to feed a DC-voltage intermediate circuit, faults in the feeding mains system can lead to fault disconnections and even to destruction of the frequency converter.
By way of example, voltage dips in the feeding mains system to the frequency converter cause torque dips at the motor which is fed from the frequency converter. If the mains voltage returns with a steep gradient, this can lead to overvoltages in the DC-voltage intermediate circuit in the frequency converter, or to motor-side overcurrents. The rectifier diodes in the mains-side converter in the frequency converter are severely loaded by rapid recharging of the intermediate-circuit capacitor in the frequency converter. If voltage dips occur with the voltage subsequently returning with a steep gradient repeatedly and successively, then the rectifier diodes will be destroyed as a result of excessive temperature, resulting in failure of the frequency converter. Very high mains voltages can lead to overvoltage disconnection or to destruction of the frequency converter.
Until now, a frequency converter which is connected to a feeding mains system has been switched off when a permissible upper limit value or lower limit value for the intermediate-circuit voltage in the frequency converter is overshot or undershot, or the motor current becomes excessive. In some commercially-available frequency converters, the failure of one mains system phase of the feeding mains system is also identified, and is used as a disconnection criterion.
Since there may be many reasons for overvoltage and overcurrent fault messages and the fault is in general not directly reproducible, it is difficult to identify the mains voltage of the feeding mains system as a trigger for disconnection of the frequency converter. If a frequency converter has failed repeatedly with an overvoltage or overcurrent fault message and there is no application-specific or equipment-specific explanation for this, the service technician will record and then evaluate the mains voltage of the feeding mains system by means of a plotter, in particular a long-term plotter.
Since an unstable mains system always represents a risk to reliable operation of a frequency converter, and thus to an installation, it would be very helpful for an operator of an installation with motors that are fed from frequency converters for each frequency converter to be able to establish the quality of its feeding mains system and to emit an appropriate message, on reaching a hazard potential. In some circumstances, this would make it possible to prevent costly interrupts in operation.